Applications requiring high wear resistance, particularly at conditions of high temperature, pressure, velocity and/or chemically degrading or corrosive, require materials that can perform for long times under stress and at high temperatures. In the aerospace market, examples of such applications are aircraft engine parts and aircraft wear pads. In the automotive market, examples of such applications are automatic transmission bushings and seal rings, tenter frame pads and bushings, material processing equipment parts, and pump bushings and seals.
Typically, a component in applications as described above is intended to function as a sacrificial, or consumable, component, preventing or reducing the wear or damage that a more costly mating or adjacent component would experience if it were mated against some other component over time under stress or oxidative stress. A component loses efficacy as a sacrificial wear reducer. However, as the component wears, the resulting increased clearances can result in deleterious effects, such as increased leakage (of air pressure or fluid) or increased noise, thereby reducing the operating effectiveness of the entire system in which the worn component is contained. Ultimately it loses its ability to prevent or reduce the wear or damage to a more costly mating or adjacent component. Restoring the system to its original operating effectiveness would require replacement of the worn component with a new un-used component. Replacement may require disassembly, reassembly, testing and re-calibration (“service”) of the system, resulting in considerable costs in terms of down-time and labor. Thus, a component that demonstrates a lower rate of wear is desirable to reduce the frequency of replacement, thereby reducing cost.
When choosing among candidate materials for a particular application, it would be useful to predict which candidate is most likely to yield parts with the longest wear life. Evaluating and comparing the wear life of parts made of different materials, or made of the same material in different manners, for these high temperature, high wear applications is difficult. Practitioners in this field typically use wear rate, such as described in the ASTM standard G133, and, where applicable, thermal oxidative stability (TOS) measurements to compare two materials.
A limitation of the thermal oxidative stability test is that it provides information on weight loss over time at temperature but does not provide direct information on wear life at temperature. Designers of jet engines draw inferences from the TOS information, reasoning that the higher the weight loss in the VV TOS test, the shorter the life time at temperature is likely to be. Likewise, wear tests provide information for designers on the wear rate of materials but only over limited periods of time, forcing designers to draw inferences about changes in wear rate as a function of time at temperature. However, the relationship among wear rate, thermal oxidative stability, and wear life is not well-defined. It is possible for two materials to have similar thermal oxidative stability values and similar wear rates as determined by ASTM G133 but very different wear life or lifetimes. Other practitioners thermally age parts or specimens made of the materials of interest and then measure the change in mechanical properties. Still others thermally age materials and then measure the change in thermal oxidative stability values. Neither of these approaches provides a direct comparison of wear life.
There remains a need for a convenient method that provides a direct, reliable comparison of the relative wear life of materials intended for use in applications requiring high wear resistance at high temperatures.